1. Field of the Invention
This invention relates to a substrate structure for a composite semiconductor device suitable for the formation of a plurality of functional elements which require element isolation.
2. Description of the Prior Art
In a composite semiconductor device having a plurality of active elements or passive elements integrated on a single substrate it is necessary to electrically isolate such elements from other elements. An isolation method for this purpose includes, for example, a method utilizing a reverse-biased PN junction, or a method utilizing an insulating material.
A semiconductor substrate having areas separated by a PN junction will be explained below by way of example.
An N type epitaxial layer is deposited on a P type semiconductor substrate, and P.sup.+ type impurity is diffused into an epitaxial layer to form an element isolation area reaching the semiconductor substrate. In this way an island-like element area is obtained which is surrounded by that PN junction. The element area is electrically isolated from other epitaxial layer portions by a depletion layer when a reverse bias is applied to the PN junction. This system offers an advantage of being lower in cost. When, on the other hand, a P.sup.+ type element isolation area is formed, diffusion unavoidably occurs in a lateral direction to an extent substantially equal to that to which it occurs in a depth direction, causing an increase in element isolation area. In the PN junction isolation a reverse bias is used in which case, since the P.sup.+ type element isolation area is usually grounded, an N type layer in the element area adjacent to the isolation layer needs to be held normally at a positive potential. Thus a bias circuit in an integral circuit which is formed within the element area is restricted. Where, for example, transistors of different conductivity types are to be formed, it will be necessary to provide a very complex bias circuit. Furthermore, in the PN junction isolation a parasitic element is usually liable to be formed. Where a transistor is formed, for example, with emitter and base layers formed in the element area and with a semiconductor substrate as a collector, a parasitic transistor can be formed due to the presence of the emitter layer, base layer and P.sup.+ type element isolation area.
The element isolation method using an insulating layer will be explained below with respect to FIG. 1.
In a plurality of N type semiconductor element areas, island-like areas are formed which are separated by oxide silicon film 2 and polysilicon layer 3. This method obviates the necessity of providing a reverse bias circuit which is required in the PN junction separation. Furthermore, a restriction caused by the parasitic element can be alleviated. However, since in this system the substrate is formed of a polysilicon, a thicker substrate is needed, which presents an economical disadvantage. It is also noted that, in a completed device, the reverse surface of the substrate cannot be employed as a current passage because the lower portion, including reverse surface of the substrate, is insulated.